1. FIELD OF THE INVENTION
The present invention relates to release liners for pressure-sensitive adhesive carrying substrates, and more particularly, to a method and apparatus for forming a compression scored release liner to permit the liner to be easily removed from the adhesive carrying substrates.
2. PRIOR ART
Many apparatus and methods have been devised for continuously or intermittently scoring a release liner for use with adhesive carrying substrates in order to permit the liner to be easily removed from the substrate immediately prior to its being used. Such apparatus generally utilize a roller which is secured to existing machinery for performing an auxilliary operation, or is used in a separate operation in the series of steps required to produce adhesive carrying substrates removably secured to a liner. This is most often accomplished in a continuous process in which a web of liner is first scored and then assembled with adhesive and substrate and the assembly is then cut to the desired dimensions of individual sheets or left in roll form. It is desirable that the scoring of the release liner does not produce a slit or cut entirely through the liner, since it would permit adhesive to seep into or through the liner.
The industry terms such release liners as "splitless" release liners. Early examples of such release liners are shown in Starr (U.S. Pat. No. 2,319,272) and Brady (U.S. Pat. Nos. 2,434,545 and 2,681,732). These patents disclose using abrading means or scoring means to reduce the cross-sectional area at the score line. Another patent which discloses a crushing-cutting rotary tool to form a scored liner is Karn U.S. Pat. No. 3,230,649. In Karn the scored areas are disrupted, evidently in order to guarantee the integrity of the liner prior to cracking along the score line.
A specific apparatus for forming such an interrupted score line is shown in Morgan U.S. Pat. Nos. 3,570,337 and 3,900,654. That apparatus utilizes a long cylindrical roller provided with a spirally wound wire or series of wires secured to the surface of the cylinder with intermittently relieved portions which cause intermittent scoring or cutting of the liner as the device is rolled over the surface of the liner.
It should be noted that devices of this nature are driven by contact with the web rather than by independent drive means due to the angular contact of the spirally wound wire with the web. The web is usually passed between the scoring roller and a separate parallel roller disposed beneath the web for supporting the liner as the roller compresses the material of the liner in the region being scored. This arrangement generally provides sufficient frictional engagement between the scoring roller and the liner to provide sufficient driving force for rotating a scoring roller without auxilliary means.
One difficulty associated with this type of prior art device is that the wire which is spirally wound around the cylindrical roller occasionally breaks or is otherwise damaged. This requires the shutting down of the entire machine, since the wire extends the length of the roller and thus will no longer score the liner at the proper locations along the width of the web.
An alternative to such a device is suggested by U.S. Pat. Nos. 3,719,548 to Keck and 3,859,157 to Morgan in which a cylindrical roller is disclosed with scoring ribs secured to the surface of the cylindrical roller concentric therewith. This permits continuous scoring of the liner. Again, the possibility exists of damaging the scoring ribs which would require machine shut down and remachining of the cylinder.
In Keck, individual rolls are suggested. The advantage of this particular construction is that a plurality of these scoring rollers may be utilized on a common axis of rotation extending across the web, so that if a single roller becomes defective the remaining rollers will score the web at the desired location. In addition, it is not necessary to replace the entire scoring roller assembly since a single element may be quickly replaced at substantially reduced cost as compared to those scoring rollers which are a single element extending across the entire web.
The Keck type of device also utilizes a pair of spaced resilient members disposed on each side of the scoring rib in spaced relation thereto and concentric therewith, for the purpose of maintaining frictional engagement with the web in order to rotate the roller by contact with the web without the use of auxilliary means.
However, such a device causes lack of uniformity in the scoring depth in the liner due to nonuniformity in the liner thickness and to wear of the resilient members over extended periods of use. This lack of uniformity can create undesirable defects in the release liner such as causing score lines which are either too deep and actually result in slitting of the release liner entirely through, or are too shallow resulting in a release liner which will not fracture in the desired manner when bent.